The transmission of information between a sender and a receiver is now largely handled and processed in a digital form--i.e., the signal information is represented as a sequence of discrete values, even though the original state of the information may have been in an analog form. Where the original information is intrinsically analog (e.g., human speech) a conversion of that analog information to a digital form is typically carried out by sampling the analog waveform at discrete intervals, and quantizing the sampled analog information into a finite set of discrete values (typically represented as binary "words").
In the transmission of digital information, sets of such discrete values comprising a small time increment of the original signal are usually grouped together into packets. It is also normally the case that the packets corresponding to an information signal from a particular source will be interspersed with packets from competing information sources (rather than all of the packets for a given information source being transmitted as a continuous stream). Some applications also require that a strict timing relationship be maintained in a transmission medium between information packets belonging to the same source and destination pair.
The field of wireless telephony provides a useful example of problems arising from the necessity to manage the sequencing or timing of information packets from competing sources over a common transmission medium. It is noted that, for purposes of this example, for each packet connection, either the information source or the destination may be a mobile unit of a wireless system.
Many wireless communications systems utilize the digital medium for transmission and processing of information from a source to a destination receiver. Moreover, as older analog mobile units are phased out over the next several years, essentially all wireless communication is expected to become digital. For such digital wireless systems, various "multiple access" methods have evolved to accommodate the multiple sources which must share a common portion of the wireless frequency spectrum. The principal such methods are known as Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA). All of these methods are well-known in the art of wireless communication. Nonetheless, as the illustrative example described herein is based on use of CDMA technology, some of the salient characteristics of CDMA will be briefly described hereafter.
With Code Division Multiple Access, multiple users share the same frequency band at the same time. Unique channels are created by having each user directly modulate its information signal by a unique, high-bit-rate code sequence that has minimal correlation with that assigned to any other user. At the other end of the RF transmission path, that modulated information signal is demodulated to recover the actual information signal.
For purposes of the example, consider a CDMA information signal transmitted over an air interface between a mobile station and an RF receiver at a base station. The received RF signal is then demodulated at the base station and the demodulated signal transmitted--usually over a wired transmission facility--to a switching/processing center serving that and other base stations, and which is usually physically separated from the base stations. The transmission facility between the base station and the switching/processing center, denoted hereafter as a packet transmission facility (PTF), is sized to accommodate the anticipated packet traffic from the mobile stations served by the base station, subject to a specified packet loss and delay requirement. At the switching/processing center, packets from each served mobile station are then identified from among the multiplicity of such packets traversing the PTF and reassembled into the specific information signals transmitted by each such mobile station. Such reassembled information signals are then sent on to the ultimate destination of the signal, generally via the Public Switched Telephone Network (PSTN) or a Public Data Network (PDN).
CDMA systems (as well as other such systems) operate pursuant to various "standards" which are established to promote compatibility among CDMA equipment provided by different suppliers for such equipment. One of these CDMA standards is designated as the IS-95A CDMA Common Air Interface, which is established under the aegis of the Telecommunications Industry Association TR 45.5 subcommittee. In an IS-95 CDMA wireless system, packets on the air interface are referred to as "frames", with each frame containing 20 msec of source information. The number of information bits in a given CDMA frame varies according to the level of speech activity represented by that frame--i.e., active speech will be represented by more bits per frame than will silence.
With many CDMA systems, mobiles acquire their timing from the base stations, with the base station transmission providing the timing reference. (Base stations in turn normally acquire their timing via the Global Positioning Satellite (GPS) system).
A problem that may arise from this arrangement is that all mobile stations could transmit their frames at substantially the same time, and, in that circumstance, all such frames would arrive at the base station, for transmission via the PTF to the switching/processing center, at substantially the same time (differing only by propagation time differences for the signals from the served mobiles). Absent the use of a PTF with a very high bandwidth (an uneconomic choice), packets may experience substantial queuing delay and this is known to have a deleterious effect on speech quality.
To address this problem, the IS-95A CDMA Common Air Interface standard permits different mobile stations to offset their frame transmissions in 1.25 msec intervals from the 20 msec timing reference. IS-95A specifies sixteen evenly spaced offsets of 1.25 msec each. With such offsets, it becomes possible to stagger frame starting times among the served mobile stations so that the queuing times of the packets from those mobiles is reduced.
A common choice in the present art is to have four evenly spaced offsets, beginning, respectively, at 0, 5, 10 and 15 msec from the timing reference. Such an arrangement, for a single frame, is illustrated in FIG. 1, where the interval between 0 and 5 msec. is designated as "skew group" 0, the interval between 5 and 10 msec. as skew group 1, and so on. The call assignment status for each skew group is illustrated symbolically by vertical compartments framing each such skew group. For each such skew group so illustrated, calls in progress from served mobiles are indicated by "X"s. Thus skew group 0 is seen to have 3 calls assigned, skew group 1: 4 calls, skew group 2: 3 calls, and skew group 3: 4 calls. It can also be seen that, with this prior art methodology, the maximum number of mobiles assigned to a given skew group (4 for the illustrated case) is one quarter of the total number of mobiles which may be handled by the PTF.
The PTF bandwidth is sized to carry a prescribed number of maximum-sized packets in a 20 msec interval. In the prior art, the maximum number of mobiles allowed to share a skew group is such that the sum of their packet transmission times over the PTF will not exceed the offset interval--for the illustrated case of four skew groups, that offset interval being 5 msec. Thus, as illustrated in FIG. 1, there is no "spillover" of packets from one skew group to another, and the maximum delay experienced by a packet is simply the offset interval (5 msec).
A particularly important attribute of CDMA wireless technology is that of the "soft" handoff, as contrasted with the "hard" handoff which characterizes most other wireless communication systems. In such a "hard" handoff, an actual break (albeit momentary) in the signal path occurs as the mobile unit is handed off from the first base station to the second base station. Under the CDMA "soft handoff" procedure, however, the mobile station commences communication with the second base station (and possibly additional base stations) without interrupting communication with the first base station--i.e., the mobile station communicates with multiple base stations simultaneously.
In this soft-handoff mode, at each frame interval, all base stations with which the mobile is communicating receive the mobile's transmitted frames. A "frame selector" is deployed at the switching/processing center to select the highest quality replica among the multiple copies of the frame received at the several base stations communication with the mobile station during that frame interval. This imposes a constraint that these multiple replicas of the frame must arrive at the frame selector at substantially the same time. In order to satisfy this constraint during soft handoff, the mobile unit must be assigned to the same skew group at all of the base stations with which it is communicating. This leads to a potential problem under the prior-art skew group procedure.
When a mobile station seeks to establish a soft-handoff with a second base station, its call must, as noted above, be assigned to the same skew group as was assigned at the first base station. However, should that second base station already have the maximum number of calls assigned to the required skew group (based on the assignment criteria previously described), a soft handoff to that second base station would be blocked. Such blocking of the soft handoff for that mobile station not only impairs signal quality for that mobile station, as it travels increasingly further from its serving first base station, but also impacts the signal quality of other mobile stations served at the second base station. This occurs because the mobile station denied a soft handoff must increase its output power to maintain an acceptable signal quality with the first base station, which in turn is likely to cause interference for the other mobile stations served by the second base station.